1. Technical Field of the Invention
The present invention relates to an apparatus and method of recognizing objects which are present outside a vehicle, and in particular, to the apparatus and method that use a technique for transmitting, from the vehicle, electromagnetic waves toward a predetermined field extending in both a widthwise direction (transverse direction) and a heightwise direction (direction vertical to a road surface) of a vehicle, and for recognizing objects based on the reflected electromagnetic waves.
2. Related Art
An object recognition apparatus has been under development, in which transmission waves, such as laser beams or millimeter waves, are emitted and an object in the forward direction of the vehicle concerned is recognized by detecting the reflected waves. Such an apparatus is applied, for example, to a device for detecting an obstacle, such as a preceding vehicle, to raise an alarm, or to a device for controlling vehicle speed such that a distance between a preceding vehicle and the vehicle concerned is maintained. Thus, the object recognition apparatus is used for recognizing an object, such as a preceding vehicle.
In performing such object recognition, it is expected that measured-distance data obtained from objects other than an object to be controlled are removed, for example, to enhance the accuracy of recognition. In this regard, JP-B-3994941 suggests a method for recognizing the distance data regarding a road surface by distinguishing it from the distance data regarding an obstacle (three-dimensional object), such as a preceding vehicle.
This method uses a mechanism that changes the mounting angle of a mechanism, per se, that emits transmission waves and receives reflected waves, as shown in JP-B-3994941. With this method, the angle of emitting transmission waves is vertically changed to determine whether or not a received distance data corresponds to a reflection from a road surface that is not a three-dimensional object.
Specifically, in performing detection, transmission waves are emitted in the widthwise direction of the vehicle in a normal state where the optical axis is substantially horizontal (normal scan mode). When a target does reside in the forward direction, transmission waves are emitted subsequently with the optical axis being directed downward (downward scan mode). Then, if the detected distance of the target changes in a manner the target approaches the vehicle, it is determined that the reflected datum is of the road surface, as shown in JP-B-3994941.
More specifically, the method disclosed in JP-B-3994941 is based on the following concept. That is, regarding the distance data obtained from a three-dimensional object, such as a preceding vehicle, the object will be detected at a position of substantially the same distance, even when the direction of the optical axis is changed from the substantially horizontal direction to the downward direction. On the other hand, regarding the distance data obtained from a road surface as an object, the distance data will change in a manner the object (road surface) approaches the vehicle. Thus, a three-dimensional object can be distinguished from a road surface.
However, the object recognition technique disclosed in JP-B-3994941 is originally based on an object recognition technique of one-dimensional scan in which transmission waves are emitted in the widthwise direction of the vehicle. Therefore, in order to determine a distance data as being of a road surface, the direction of the optical axis has to be changed downward until the direction matches an angle that enables acquisition of a distance data corresponding to the reflection from the road surface such that it is not a three-dimensional object.
Meanwhile, an object recognition technique using two-dimensional scan has been known, which does not entail such a change of the mounting angle of a mechanism, per se, for the optical axis matching. In this technique, beams are emitted along scan directions, i.e. widthwise and heightwise directions of the vehicle. However, the object recognition technique using two-dimensional scan is based on a premise of recognizing three-dimensional objects, such as a preceding vehicle or a roadside object, and thus is not configured to determine whether or not a distance data in question corresponds to the reflection from a road surface.
Therefore, the object recognition technique using two-dimensional scan is not able to distinguish whether or not an acquired datum is the reflected datum obtained from an object (three-dimensional object) or the reflected datum obtained from a road surface. Accordingly, with this technique, a reflected datum is likely to be erroneously recognized as being the reflected datum obtained from an object in spite of the fact that the reflected datum is of a road surface.
Specifically, in recognizing an object in the forward direction of the vehicle by emitting transmission waves, such as laser beams or millimeter waves, and detecting the reflected waves, reception signals as the reflected waves include information mixing up both the information on a road surface and the information on an object. Therefore, it is desirable that these pieces of information are distinguished from each other in the object recognition technique using two-dimensional scan as well. If such a distinction is made, an object will be more correctly recognized.